The following brief description of a cathodic protection system illustrates an application and environment in which the measurement of a small DC voltage is equally challenging and necessary. Cathodic protection systems are designed to combat corrosion that inevitably occurs when metallic structures are located in contact with the soil or another electrolytic medium. The metallic structure is typically coated to retard corrosion. Coatings for large structures such as steel pipelines inevitably include or develop defects, voids, or “holidays” that expose the underlying metal to the medium. To address corrosion that would otherwise occur at a holiday, a cathodic protection (CP) system impresses an electrical current in opposition to a corrosion current that flows between the metallic structure and the electrolytic medium. An impressed current CP system includes an external DC power supply that biases the metallic structure relative to the electrolytic medium, changing the electrochemical state of the metallic structure and thereby preventing or slowing the corrosion process.
The voltage across the interface between the metallic structure and the electrolytic medium is a primary determinant of how well a CP system protects a pipeline or other structure against corrosion. For CP system applications in which the metallic structure is an underground steel pipeline and the electrolytic medium is the surrounding soil, the “pipe-to-soil” potential (PTSP), i.e., the potential across the interface between the pipe and soil, is a critical parameter. One rule of thumb commonly encountered in the cathodic protection industry suggests a pipe-to-soil potential more negative than −0.85 V.
Direct measurement of the PTSP not being generally feasible, the PTSP may be estimated by measuring the potential between the pipe and a reference electrode, e.g., a copper-copper sulfate electrode, located in proximity to the pipe itself. This technique, however, introduces an unwanted resistive element, corresponding to the soil between the pipe and the reference electrode, into the current path of the PTSP measurement circuit. An IR voltage drop across this resistive element represents a discrepancy between the measured potential and the actual PTSP. The magnitude of this discrepancy may be estimated by measuring a first voltage, referred to as the “on potential”, while the CP system is operating and a second voltage, referred to as an “instant off potential”, very soon after turning off the CP system under the assumption that the primary component of any difference between the on potential and the instant off potential is attributable to the elimination of the IR drop across the soil when the impressed current is terminated.
Cathodically protected structures may be located in proximity to one or more sources of time varying electromagnetic fields. Steel pipelines, for example, are frequently located beneath or in proximity to high power transmission lines. In such environments, the amplitude of the total noise, which may include thermal noise, noise induced by these transmission lines, and noise attributable to other sources, may exceed the amplitude of the signal-of-interest by multiple orders of magnitude, thereby making accurate estimates of the PTSP difficult to obtain.